Present reserves of fossil fuel result from the storage of ancient solar energy as chemical energy. As these reserves are combusted, the attendant growth in the atmospheric and oceanic reservoirs of CO.sub.2 contributes to the greenhouse effect. There has been a general recognition of the need for an efficient process to assist natural processes in photochemically removing CO.sub.2, while storing solar energy. Ongoing developments in solar fuels involve either biological processes or hydrogen production from water splitting. No known research addresses the problem directly through the physical and spectral properties of CO.sub.2. It has been noted in the literature that CO.sub.2 has no spectral absorption in the visible or near-ultraviolet portion of the spectrum under normal circumstances; therefore, direct photoreduction of CO.sub.2 has appeared to be unlikely and has received little attention.
FIG. 1 hereof, labeled prior art, shows existing measurements of the temperature dependence of the CO.sub.2 absorption spectrum. The dotted and solid curves were reported by B. R. Lewis and J. H. Carver, in J. Quant. Spectrosc. Radiat. Transfer 30, 297 (1983), at 202 and 367 K, respectively. Some temperature dependence was observed throughout the entire spectral range, but the greatest effect was at longer wavelengths. The absorption cross sections were found to increase with temperature at about 1.5%/K at the long wavelength limit of these data sets (197 nm). K. Yoshino et al., in J. Quant Spectrosc. Radiat. Transfer 55, 53 (1996), measured the same spectrum at two temperatures (195 K and 295 K) between 120 and 175 nm. Their results were nearly identical to those of Lewis and Carver, supra. These results suggest that substantial enhancement of absorption cross section may be expected at longer wavelengths and elevated temperatures. The spectrum published by J. W. Rablais et al. in Chem. Rev. 71, 73 (1971) and the measurement of D. E. Shemansky in J. Chem. Phys. 56,1582 (1972) agree with the room temperature data of Lewis and Carver, supra, in the region of overlap. Shemansky, supra, extended the measurement out to 300 nm where extinction by Rayleigh scattering interfered with observation of the very weak absorption.
M. Koshi et al. in Chem. Phys. Lett. 176, 519 (1991) reported a dramatic increase in the absorption of CO.sub.2 with temperature between 1500 and 3000 K. The absorption cross sections at 193 nm were inferred in shock-heated CO.sub.2 by measuring the atomic oxygen produced by photolysis of CO.sub.2. The determination of absorption cross sections assumed a unity photolysis quantum yield to O(.sup.3 P) atoms. Measurements at 1520 K and 2850 K are also shown in FIG. 1 as circles and squares, respectively. N. A. Generalov et al. in Opt. Spectrosc. 15, 12 (1963) measured the absorption of CO.sub.2 behind a shock wave at temperatures as high as 6300 K for wavelengths of 238 and 300 nm. Absorption out to 355 nm at 5000 K was observed. Although the reported data are not precise, an increase of absorption with temperature is indicated. Cross sections and error estimates were derived by the present inventors from the data at 1523 K, 1818 K, 2073 K, and 2273 K. The results for 2273 K are shown as triangles in FIG. 1.
The visible emission seen by many in carbon monoxide/oxygen flames is evidence that transitions exist for CO.sub.2 to absorb visible and near ultraviolet light if the molecule is heated sufficiently. This emission has been studied between 310 and 380 nm in R. N. Dixon, Proc. Roy. Soc. 275, 431 (1963), and the conclusion was drawn that it came from transitions between the bent .sup.1 B.sub.2 state of CO.sub.2 to highly excited vibrational states of the electronic (.sup.1 .SIGMA..sub.g.sup.+) ground state. Thermal population of the highly excited vibrational states at high temperatures should allow absorption, the reverse of this emission process, to occur. To reach the absorption transitions observed by Dixon, supra, in emission would require vibrational energy in excess of 1.88 eV. At room temperature, the fraction of molecules with more than 1.88 eV is 7.times.10.sup.-30. However, that fraction is 1.3.times.10.sup.-4 at 1523 K and 8.8.times.10.sup.-3 at 2273 K. Transitions to the .sup.1 A.sub.2 state may also contribute to absorption by CO.sub.2. C. Cossart-Magos et al., in Mol. Phys. 75, 835 (1992) have attributed nine weak bands between 175 and 200 nm to transitions to that state. Similar transitions from vibrationally excited states could contribute to the absorption spectrum at longer wavelengths and elevated temperatures.
Direct solar reduction of CO.sub.2 would require significant absorption cross section beyond 300 nm as is illustrated in FIG. 1, where a portion of the solar irradiance at the earth's surface taken from Solar and Terrestrial Radiation: Methods and Measurements by K. L. Coulson, Academic Press, New York (1975), p. 40, is shown.
Accordingly, it is an object of the present invention to provide a process for the direct solar reduction of CO.sub.2 to CO.
Another object of the present invention is to provide a process for generating CO and hydrogen for liquid fuel production from CO.sub.2 in the ambient atmosphere.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.